Trophic interrelationships and community structure

Although food webs in tropical waters are often very complex, they may be based on relatively few sources, for example the aufwuchs which supports numerous species on the rocky shores of Lake Malawi (Fig. 4.2), or the benthic algal flora that supports herbivorous fishes on marine reefs. In any food chain there are rarely more than four or five links; long chains are energetically expensive as a large proportion of the potential energy is lost at each successive stage. In freshwaters alternate chains run: (1) from bottom detritus, through microorganisms, to detritus–feeding invertebrates or fish, to several levels of piscivore; or (2) in the pelagic zone from phytoplankton to zooplankton, to zooplanktonfeeders, then to one or more levels of piscivore. In river systems the detrital chain is more important, based largely on allochthonous materials in the headwater streams while in lower reaches detritus comes mainly from decomposition of aquatic macrophytes (see the ‘river continuum concept’ of Vannote et al., 1980; but see also Rzoska, 1978). As a lake forms, the pelagic plankton chain becomes increasingly important, as seen after the filling of man–made lakes, and also in lakes such as Chad, populated with riverine fishes such as Alestes baremose which changed its riverine diet of insects and seeds to exploit zooplankton in the lake.